Japanese patent application laid-open No.63-314502(1988) discloses a diffraction-utilizing polarization beam splitter as shown in FIGS. 1 and 2A. In this polarization beam splitter, proton exchanging regions 22 with rectangular cross sections are periodically formed on the surface of a lithium niobate substrate 21 which has an in-plane optical axis 4, and phase compensating films 23 with rectangular cross sections are periodically formed on the proton exchanging regions 22. A component(ordinary light component) polarized vertically to the optical axis 4 of incident light 5 supplied from the lower part in FIG. 1 is transmitted through the polarization beam splitter to give transmitted light 6. On the other hand, a component(extraordinary light component) polarized parallel to the optical axis 4 of the incident light 5 is diffracted by the polarization beam splitter to give +1st-order diffraction light 7 and -1st-order diffraction light 8. The phase compensating film 23 is of, for example, Nb.sub.2 O.sub.5.
Japanese patent application laid-open No.6-27322(1994) discloses another diffraction-utilizing polarization beam splitter as shown in FIG. 2B. In this polarization beam splitter, proton exchanging regions 32 with rectangular cross sections are periodically formed on the surface of a lithium niobate substrate 31, and grooves 33 with rectangular cross sections are periodically formed on the proton exchanging regions 32. A component(extraordinary light component) polarized parallel to its optical axis of incident light is transmitted through the polarization beam splitter to give transmitted light. On the other hand, a component(ordinary light component) polarized vertically to the optical axis of the incident light is diffracted by the polarization beam splitter to give +1st-order diffraction light and -1st-order diffraction light.
In FIGS. 2A and 2B, the following symbols are defined.
.lambda.: a wavelength of incident light PA1 .DELTA.n.sub.o : a refractive-index variation of ordinary light by proton exchanging PA1 .DELTA.n.sub.e : a refractive-index variation of extraordinary light by proton exchanging PA1 n.sub.s : a refractive index of the lithium niobate substrate 21 or 31 PA1 n.sub.p : a refractive index of the phase compensating film 23 PA1 h.sub.5 : a depth of the proton exchanging region 22 PA1 h.sub.6 : a thickness of the phase compensating film 23 PA1 h.sub.7 : the sum of a depth of the proton exchanging region 32 and a depth of the groove 33 PA1 h.sub.8 ; a depth of the groove 33 PA1 .lambda.: a wavelength of incident light PA1 n.sub.Q : a refractive index of the glass 41 PA1 n.sub.qo : a refractive index of ordinary light to the quartz 42 PA1 n.sub.qe : a refractive index of extraordinary light to the quartz 42 PA1 h.sub.g : a height of the saw-toothed part PA1 a lithium niobate or lithium tantalate substrate which has an in-plane optical axis; PA1 proton exchanging regions formed periodically on a surface of the substrate; and PA1 phase compensating film regions formed periodically on the proton exchanging regions; PA1 wherein each of the proton exchanging regions has a step cross section with 2.sup.n -step depths and each of the phase compensating film regions has a step cross section with 2.sup.n -step thicknesses corresponding to the depths of the each proton exchanging region, where n is an integer of 2 or more. PA1 a lithium niobate or lithium tantalate substrate which has an in-plane optical axis; PA1 proton exchanging regions formed periodically on a surface of the substrate; and PA1 grooves formed periodically on the proton exchanging regions; PA1 wherein each of the proton exchanging regions and each of the grooves have a step cross section with 2.sup.n -step depths, where n is an integer of 2 or more. PA1 forming periodically first proton exchanging regions with a depth of h.sub.a on a lithium niobate or lithium tantalate substrate which has an in-plane optical axis; PA1 forming repeatedly and sequentially n-th proton exchanging regions with a depth of h.sub.a /2.sup.n-1 on a part of the substrate or proton exchanging regions; PA1 forming periodically first phase compensating film regions with a thickness of h.sub.b on proton exchanging regions; and PA1 forming repeatedly n-th phase compensating film regions with a thickness of h.sub.b /2.sup.n-1 on a part of the proton exchanging regions or phase compensating film regions; PA1 wherein n is an integer of 2 or more. PA1 forming periodically first proton exchanging regions with a depth of h.sub.a on a lithium niobate or lithium tantalate substrate which has an in-plane optical axis; PA1 etching by a depth of h.sub.o the top surface of the proton exchanging regions to form first grooves; and PA1 repeatedly forming n-th proton exchanging regions with a depth of h.sub.a /2.sup.n-1 while masking a part of the substrate or proton exchanging regions and etching by a depth of h.sub.o /2.sup.n-1 the top surface of the n-th proton exchanging regions to form n-th grooves; PA1 wherein n is an integer of 2 or more.
In case of .lambda.=685 nm, .DELTA.n.sub.o =-0.04, .DELTA.n.sub.e,=0.12 and n.sub.s =2.2 are obtained, and, in case of the phase compensating film 23 of Nb.sub.2 O.sub.5, n.sub.p =2.2 is obtained.
In FIG. 2A, phase differences .phi..sub.o and .phi..sub.e between light to be transmitted through part where the proton exchanging region 22 and phase compensating film 23 are formed and light to be transmitted through part where these are not formed, to ordinary light and extraordinary light, respectively, are given by: EQU .phi..sub.o =(2.pi./.lambda.).multidot.[.DELTA.n.sub.o h.sub.5 +(n.sub.p -1)h.sub.6 ] (1) EQU .phi..sub.e =(2.pi./.lambda.).multidot.[.DELTA.n.sub.e h.sub.5 +(n.sub.p -1)h.sub.6 ] (2)
In FIG. 2B, phase differences .phi..sub.o and .phi..sub.e between light to be transmitted through part where the proton exchanging region 32 and groove 33 are formed and light to be transmitted through part where these are not formed, to ordinary light and extraordinary light, respectively, are given by: EQU .phi..sub.o =(2.pi./.lambda.).multidot.[-.DELTA.n.sub.o (h.sub.7 -h.sub.8)+(n.sub.s -1)h.sub.8 ] (3) EQU .phi..sub.e =(2.pi./.lambda.).multidot.[-.DELTA.n.sub.e (h.sub.7 -h.sub.8)+(n.sub.s -1)h.sub.8 ] (4)
Furthermore, transmittances .eta..sub.0o, .eta..sub.0e and +1st- and -1st-order diffraction efficiencies .eta..sub.1o, .eta..sub.1e to ordinary light and extraordinary light, respectively, of the polarization beam splitter are given by: EQU .eta..sub.0o =cos.sup.2 (.phi..sub.o /2) (5) EQU .eta..sub.0e =cos.sup.2 (.phi..sub.e /2) (6) EQU .eta..sub.1o =(4/.pi..sup.2)sin.sup.2 (.phi..sub.o /2) (7) EQU .eta..sub.1e =(4/.pi..sup.2)sin.sup.2 (.phi..sub.e /2) (8)
In FIG. 2A, in case of .phi..sub.o =0 and .phi..sub.e =.pi., ordinary light is perfectly transmitted through and extraordinary light is perfectly diffracted since .eta..sub.0o =1 and .eta..sub.0e =0 are obtained. In this case, h.sub.5 =2.14 .mu.m and h.sub.6 =71.4 nm are obtained by equations (1) and (2) Also, .eta..sub.1e =0.405 is obtained.
In FIG. 2B, in case of .phi..sub.o =.pi. and .phi..sub.e =0, extraordinary light is perfectly transmitted through and ordinary light is perfectly diffracted since .eta..sub.0o =0 and .eta..sub.0e =1 are obtained. In this case, h.sub.7 =2.35 .mu.m and h.sub.8 =214 nm are obtained by equations (3) and (4). Also, .eta..sub.1o =0.405 is obtained.
As explained above, in the conventional polarization beam splitters shown in FIGS. 2A and 2B, both the +1st- and -1st-order diffraction efficiencies of the polarization components diffracted by the polarization beam splitter are 0.405. Therefore, there are problems that, when either of the +1st- and -1st-order diffraction lights is used, the efficiency is low, and that, when both the +1st- and -1st-order diffraction lights are used, the optical system is so much complicated.
Japanese patent application laid-open No.6-274927(1994) discloses yet another polarization beam splitter as shown in FIG. 3, where the efficiency is enhanced when either of the +1st- and -1st-order diffraction lights is used. In this polarization beam splitter, glass 41 as an isotropic medium and quartz 42 as an anisotropic medium are Functioned at boundary planes with a saw-toothed cross section. A component (ordinary light component) polarized vertically to the optical axis of the quartz 42 is transmitted through the polarization beam splitter to give transmitted light. On the other hand, a component (extraordinary light component) polarized parallel to the optical axis of the quartz 42 is diffracted by the polarization beam splitter to give +1st-order diffraction light.
In FIG. 3, the following symbols are defined.
According to an example in the above application, .lambda.=856.3 nm, n.sub.Q =1.5419, n.sub.qo =1.5419 and n.sub.qe =1.5509 are obtained.
In FIG. 3, transmittances .eta..sub.0o, .eta..sub.0o and +1st-order diffraction efficiencies .eta..sub.+1o, .eta..sub.+1e to ordinary light and extraordinary light, respectively, of the polarization beam splitter are given by: EQU .eta..sub.0o =sin.sup.2 .phi..sub.o /.phi..sub.o.sup.2 (9) EQU .eta..sub.0e =sin.sup.2 .phi..sub.e /.phi..sub.e.sup.2 (10) EQU .eta..sub.1o =sin.sup.2 .phi..sub.o /(.phi..sub.o -.pi.).sup.2(11) EQU .eta..sub.1e =sin.sup.2 .phi..sub.e /(.phi..sub.e -.pi.).sup.2(12)
where .phi..sub.o, .phi..sub.e be are given by: EQU .phi..sub.o =(2.pi./.lambda.).multidot.(n.sub.qo -n.sub.Q)h.sub.g /2(13) EQU .phi..sub.e =(2.pi./.lambda.).multidot.(n.sub.qe -n.sub.Q)h.sub.g /2(14)
Thus, .phi..sub.o =0 is obtained by equation (13). In case of .phi..sub.e =.pi., ordinary light is perfectly transmitted through and extraordinary light is perfectly diffracted since .eta..sub.0o =1 and .eta..sub.0e =0 are obtained. In this case, h.sub.g =72.9 .mu.m is obtained by equation (14). Also, .eta..sub.+1e =1 is obtained.
As explained above, in the conventional polarization beam splitter in FIG. 3, a high efficiency can be obtained even when only the +1st-order diffraction light is used since the +1st-order diffraction efficiency of the polarization component diffracted by the polarization beam splitter is 1.
However, the above polarization beam splitter has some problems. First, it is difficult to select a medium combination that the refractive index of an isotropic medium is equal to that of an anisotropic medium to ordinary light or extraordinary light. Second, it is difficult to form an isotropic medium and an anisotropic medium to have saw-toothed part with a height of several tens .mu.m and, further, to junction them. Third, when the grating is composed of a curved pattern or combining different patterns, the formation of the saw-toothed part itself is very difficult.